How should a dress shirt fit?

The anchors are shoulders, chest, and sleeve. The shoulder seam should sit at the edge of your shoulder, not droop onto the arm; the chest should let you pinch a couple of inches of fabric at the side without pulling; and the cuff should end at the base of the thumb so it stays put under a jacket. Collar should admit one finger comfortably. Beyond that, "slim" versus "classic" is brand-relative marketing, so match the published chest and sleeve measurements to your own and read broad user consensus on whether a shirt runs large or small — we report that as consensus, not as our own wear-testing. Length and sleeve are cheap for a tailor to fix; shoulders are not, so buy for the shoulders first.